For some pelagic fish stocks it is difficult to assess the production of young fish before they have recruited into the adult population. This is true for the northeast Arctic saithe and is because the young year-classes spends their immature years in habitats that are inaccessible for research vessels. A new Norwegian study shows, however, that the occurrence of 1-yr old saithe in European shag diet provides good estimates of the year-class strength of 3-yr old saithe two years later. It is suggested that these results can be used as a tool for an adaptive management of the fishers of young saithe.

Pelagic fish populations in Norwegian waters are monitored by scientific trawling from research vessel. Such monitoring is important to determine quotas in commercial fisheries. Knowledge about recruitment, year-class strength and the adult population size are decisive factors. In some fish stocks, such as the northeast Arctic saithe, the juveniles live in kelp forest along the coast and are inaccessible for scientific surveys. The first stock measurements cannot be made before the fish enter the pelagic population when they are 3-yr old. The European shag has, however, these youngest year-classes of saithe as its main food source. With the help of regurgitated pellets containing undigested fish bones and otoliths, a group of Norwegian scientists has looked at the opportunity to use the shags diet as an indicator of saithe recruitment.

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The European shag is a coastal, piscivorous top predator who feeds on small fish and young year-classes of larger fish species. Shags are generalists, and their diet is likely to be dominated by the most abundant prey species. The results of this study show that the occurrence of 1-yr old saithe in the shags diet is highly correlated with fishery survey abundance indices of 3-yr old saithe in the pelagic stock two years later. This means that the occurrence of 1-yr old saithe in the shags diet can be used as an early and reliable indicator of saithe recruitment two years earlier than normal.

Contact person: Svein-Håkon Lorentsen, NINA

Regurgitated pellets from European shags allow scientists to predict future levels of recruitment of young saithe.
Photo: Svein-Håkon Lorentsen